GB2268100A

GB2268100A - Turbine rotors

Info

Publication number: GB2268100A
Application number: GB9212135A
Authority: GB
Inventors: Michael James Fraser
Original assignee: Turbine Blading Ltd
Current assignee: Turbine Blading Ltd
Priority date: 1992-06-09
Filing date: 1992-06-09
Publication date: 1994-01-05
Anticipated expiration: 2012-06-09
Also published as: AU4335293A; GB2268100B; GB9212135D0; WO1993025340A1

Abstract

A turbine rotor having a plurality of blade supporting discs disposed at spaced positions along a longitudinal axis of the rotor is repaired, modified or made by providing a ring element 20 having at least one discontinuity therein providing at least one pair of separate end faces 21, 22, 23, 24, which ring element is adapted to locate with or be otherwise secured to rotor blades in an array of blades, the ring element being secured to a foundation part of the rotor by welding the ring element to said foundation part.

Description

The ring element may have a single discontinuity providing a single pair of separated end faces.

When the ring element has a single pair of end faces the ring element may be moved to said first position by moving the ring element in an axial direction relative to the rotor and the ring element is moved from said first position to said second position by moving the ring element in an axial direction relative to the rotor.

Alternatively, the ring element may have two discontinuities therein to provide two pairs of separated end faces.

When the ring element has two pairs of end faces the ring element may be moved to said first position by movement of the ring element relative to the rotor in a direction transverse to the longitudinal axis of the rotor whilst the ring element may be moved from the first position to the second position by movement relative to the longitudinal axis of the rotor.

When the ring element has two pairs of end faces disposed appropriately, for example, at opposite ends of a diameter of the ring element, the parts of the ring element may be moved directly to a position adjacent the foundation part and axially aligned therewith by virtue of movement of the ring element in a direction transverse to the longitudinal axis of the rotor.

When the ring element is disposed in said position adjacent to and axially aligned with the foundation part it may be spaced therefrom and located in said position by spacer members disposed between the ring element and the foundation part. The spacer members may comprise roller elements.

Alternatively, when the ring element is disposed in said position adjacent to and axially aligned with the foundation part it may be located therein by virtue of engagement of weld preparation surfaces of the ring element and/or foundation part.

The ring element may be made by a forging technique or a rolling technique.

Title: "Turbine Rotors" Description of Invention This invention relates to turbine rotors comprising a plurality of bladesupporting discs and to a method of repairing, modifying or making such rotors.

It is known to repair a rotor by building up a plurality of layered weldments to form a build-up on the rotor which can be machined into a bladeretaining device (US-4,958,431). Such a method suffers from the disadvantages normally associated with the relatively slow build-up of layered weldments.

An object of the invention is to provide a new and improved method for repairing a turbine rotor.

According to one aspect of the invention we provide a method of repairing, modifying or making a turbine rotor having a plurality of blade supporting discs disposed at spaced positions along a longitudinal axis of the rotor comprising, providing a ring element having at least one discontinuity therein to provide at least one pair of separated end faces, moving said ring element into a position adjacent a foundation part of the rotor and welding the ring element to the foundation part.

The separated end faces of said at least one pair of separated end faces may be joined together.

Said separated end faces may be joined together by welding or by mechanical inter-engagement or by fasteners or by any other suitable means.

The ring element may initially be moved to a first position adjacent the foundation part but axially displaced therefrom, the ring element subsequently being moved to a second position adjacent the foundation part and axially aligned therewith and the ring element being welded to the foundation part at said second position. Said separated end faces may be joined together at the first position, or at the second position or may remain not joined together.

When the ring element is made by a forging technique, so that it is forged to a complete ring, the ring element may then be machined to a desired external shape and/or to provide a desired weld preparation configuration.

When the ring element is made by a rolling technique so that it is initially formed with a single discontinuity with separable end faces the end faces may be joined together, for example, by a tack welding operation and an appropriate machining and weld preparation operation may then be performed, followed by the formation of at least one discontinuity of the ring.

When the ring element is in said first position said end surfaces may be joined together, for example by tack welding or otherwise.

The ring element may be machined to final configuration after welding to the foundation part.

The end surfaces of the or each pair may be disposed at a position at which material is to be machined away to provide a blade retaining formation.

Alternatively, where a blade is to be retained by means of fasteners such as rivets the joint may be disposed at a predetermined distance from the position at which the fasteners are to be provided.

The end surfaces may be joined together by a tungsten inert gas welding process (hereinafter TIG).

The welding of the ring element to the foundation part may be performed by a TIG welding operation and/or by a submerged arc welding operation and may be performed from one side of the ring element or from both sides with appropriately shaped weld preparation surfaces provided on the foundation part and/or the ring element.

Preferably the ring element is welded to the foundation part by performing welding operations simultaneously from both sides of the ring element and preferably these are carried out using a multi-wire TIG welder.

When a rotor is to be repaired the foundation part may be provided by machining away a relatively small radial extent of an original rotor disc, for example the part of the rotor disc providing the blade retaining means, for example "steeples" provided with axially extending grooves to engage with corresponding formations of a blade foundation.

When a rotor is to be modified, a part of the original disc is removed and a ring element of desired modified configuration is welded to the residual foundation part of the rotor, the foundation part being built up by a welding operation to a desired modified configuration.

The foundation part may be built up to the modified configuration before or after welding the ring element to the foundation part.

The removed part may be removed by mechanically removing material of the original rotor disc, which term is intended to mean any procedure for removing metal including, for example, machining, grinding, electric arc cutting or any other desired method. The extent of removal of the material of the original rotor disc is determined to minimise the possibility of creating a heat affected zone in a high stress area of the rotor disc when the ring element is welded to the foundation part.

The position of join between the ring element and foundation part is also preferably disposed in a region of relatively low stress.

If desired, all or substantially all of a disc may be removed from the rotor and the ring element in this case may have relatively a small internal opening of a diameter to engage with the relatively small diameter foundation part. In this case the foundation part may be of the same diameter as the remainder of the rotor shaft or may be of slightly greater diameter.

If desired a rotor may be originally manufactured by providing a shaft which may have foundation parts of different diameter, at least one foundation part inwardly of end foundation parts being of smaller diameter than the end foundation parts and a ring element being welded to said at least one smaller diameter foundation part as described hereinbefore.

The welding operation may be performed on a lathe on site. The lathe may be transported to site.

The welding operation may be performed by rotating the rotor about a horizontal axis or about a vertical axis.

The rotor disc may be made of any conventional material such as low alloy steel commonly containing less than 6% by weight alloying element such as Cr.Mo.V alloy (ASTM A470, Class 8). The ring may be made of similar material or may be made of other materials such as those commonly known as "super materials".

The weld metal between the separated end surfaces and the weld material between the ring element and the foundation part may be of suitable material compatible with the materials to be welded.

According to a second aspect of the invention we provide a turbine rotor when repaired, modified or made according to the first aspect of the invention.

According to a third aspect of the invention we provide a turbine rotor having a plurality of blade supporting discs disposed at spaced positions along a longitudinal axis of the rotor wherein at least one disc comprising a ring element, having at least one discontinuity therein to provide at least one pair of end faces, welded to a foundation part of the rotor.

The end faces may be joined together, for example by welding.

Alternatively, the end faces may be provided by faces of a blade retaining means such as a steeple.

The relevant invention will now be described by way of example with reference to the accompanying drawings wherein: FIGURE 1 is a diagrammatic cross-sectional view through a low pressure steam turbine rotor embodying the invention, FIGURE la is an enlarged view of the part ringed in Figure 1, FIGURE 2 is a side elevation of a ring element for use in repairing the turbine of Figure 1, FIGURE 3 is a section on the line 3-3 of Figure 1 but showing a stage prior to the completion of the repair illustrated in Figure 1, FIGURE 4 is a section on the line 4-4 of Figure 2, and FIGURE 5 is a view similar to that of Figure 4 showing another alternative weld preparation, FIGURE 6 is a view similar to that of Figure 4 and showing a still further alternative weld preparation, FIGURE 7 is a fragmentary longitudinal sectional view through a low pressure turbine rotor illustrating a second embodiment of the invention, FIGURE 8 is a section on the line 8-8 of Figure 1, and FIGURE 9 is a view similar to that of Figure 8 but showing an alternative configuration of ring element.

FIGURE 10 is a view similar to that of Figure 2 but showing an alternative configuration of ring element, FIGURE 11 is a view similar to that of Figure 2 but showing a still further configuration of ring element, FIGURE 12 is a view similar to that of Figure la but showing a rotor which has been modified by a method embodying the invention, FIGURE 13 shows a view similar to that of Figure 4 but where the ring element is welded to the foundation part by a combination of TIG and submerged arc welding, and FIGURE 14 shows an alternative configuration of rotor in which the method of the present invention may be used.

Referring to Figures 1 - 5 of the drawings, a steam turbine rotor is indicated generally at 10 and comprises a shaft part 11 from which a plurality of blade supporting discs 12a - c extend. In the illustrated example the discs 12a c are formed integrally with the shaft part 11 of the rotor but, if desired, the discs 12a - c may be a shrink fit on the shaft 11 or may be welded thereto.

The discs 12a - c are of the configuration illustrated in longitudinal cross-section and are machined to provide a plurality of steeples 13a - c of "fir tree" configuration to provide grooves which co-operate with corresponding foundation portions of blades 14.

When damage occurs to a rotor, such as the rotor illustrated in Figure 1, for example as a result of cracks forming in a steeple or because of wear, the damaged part of the disc is removed by a mechanical removal operation, for example by machining away or by grinding or by an arc cutting technique or in any other suitable manner. The residual part of the disc provides a foundation part which is illustrated at 16 in Figures 1 and la. The foundation part 16 has a radially outer surface 17 which is formed to a desired configuration. ln the illustrated example the configuration is a cylindrical surface as illustrated in Figures la and 3, but it may be formed to other suitable configuration, for example a conical surface or bi-conical surface if desired.The radial position of the surface 17 is chosen so that the heat affected zone which is formed when a welding operation is performed to join the surface 17 to another surface, as hereinafter to be described, the heat affected zone is adequately spaced from the regions of highest stress by an appropriate distance, for example about 1 cm. In addition the surface 17 is positioned so as to be located in a relatively low stress area of the disc.

A ring element 20 is made by a suitable technique, for example forging or rolling, and with appropriate machining to provide a desired external crosssectional configuration which may correspond to a final configuration or an interim configuration and with a desired weld preparation configuration. In the present example the final configuration is to correspond to the external configuration of the initial disc.

When the ring element is made by forging it is machined, after forging, to the above mentioned desired cross-sectional configuration.

Where the ring element is made, for example by rolling so that it is provided with a single discontinuity having separable end faces, the end faces are joined together by suitable means such as tack welding and then the appropriate machining operation is performed.

After machining has been completed, the ring element is provided with a desired number of discontinuities as hereinafter to be described.

Because the original diameter of the disc under discussion, i.e. the disc identified at 12a in Figure 1, is less than the diameter of the end discs identified at 12b in Figure 1, the ring element 20, as best shown in Figure 2, is provided with two discontinuities to provide two pairs of separated end surfaces 21, 22 and 23, 24 respectively. In the present example the surfaces 21 - 24 substantially extend radially of the ring but, if desired, they may be inclined to a radius and may be of other than planar configuration. The surfaces, in the present example, are formed so as to provide a generally V-shaped weld preparation feature 25.

The ring element is initially moved into a first position P1, shown in dotted line in Figure 1, adjacent the foundation part 16 but axially spaced therefrom. The two parts of the ring element are moved into this position as a result of movement of the parts of the ring element in a direction radially of the longitudinal axis L-L of the rotor. When the parts of the ring element have been thus disposed in the first position P, the surfaces 21 and 22 are joined together by performing a suitable tack welding operation or by otherwise securing them together.

The ring element, after any testing, such as X-ray inspection, and/or machining operation which may be desired, is then moved from the first position Pl, where access for testing and/or machining is facilitated, to a second position P2 adjacent the foundation part 16 but axially aligned therewith.

As best shown in Figure 2 in the second position the ring element 20 is spaced from the foundation part 16 by a gap 26 and suitable spacers such as three rollers 27 are interposed between the surface 17 of the foundation part 16 and the inner surface 28 of the ring element 20.

The ring element 20 is then welded to the foundation part 16 by performing a suitable welding operation to fill the gap 26 therebetween.

If desired the end faces may be left unjoined together until after the two parts of the ring element have been moved into the second position P2 and welded to the foundation part 16. This may be desirable in particular cases since it permits of movement of the adjacent end faces of a discontinuity towards each other during the welding of the ring element parts to the foundation part. In this case, if desired, the end faces may be subsequently welded wholly or partly together.

The welding operation may, for example, be a submerged arc welding operation and/or a TIG welding operation.

In the present example a TIG welding operation is performed from one side of the disc by virtue of the internal surface 28 of the ring element 20 being of frusto-conical configuration, as shown in Figure 4.

If desired, however, the internal surface may be of V-shaped crosssection, as shown at 28' in Figure 4, in which case a TIG welding operation can be performed from opposite sides of the disc.

Preferably, the internal surface 28" is of the configuration shown in Figure 5, namely of frusto-triangle shape in cross-section, the central part 28"a of the surface being of a diameter so as to contact the external surface 17" of the foundation part 16". In this case the welding operation is a TIG welding operation performed with two multi-wire TIG welding guns disposed on opposite sides of the disc so that two TIG welding operations are performed simultaneously. This has the advantage of providing a relatively high deposition rate and by welding from both sides simultaneously risk of distortion is minimised and by using TIG welding as opposed to, for example, submerged arc welding, a relatively fine grain structure and a stronger weld is obtained. With a, for example, three-wire gun about two or three pounds of metal per hour may be deposited.By laying down a relatively large amount of metal on each side simultaneously the total heat input is minimized as fewer total number of passes are required.

The welding operation is preferably performed with a high frequency pulse.

If desired an initial dry pass may be performed to melt the central part 28"a of the weld preparation and then build up the weld with a conventional TIG welding operation so that the centre of the weld has parent metal and weld filler metal is in the remainder of the weld.

Figure 6 shows an alternative form of weld preparation where initially the internal surface 28"' of the ring element 20"' and the outwardly facing surface 17"' of the foundation part 16"' are formed to a configuration shown in Figure 6 to form a "closed butt" weld by a TIG welding operation, preferably of the multiwire type as described hereinbefore, to build up weld metal W in the region shown. The parent metal of the ring element 20"' and foundation 16"' is then machined away as indicated by the dotted lines L and a further multi-wire TIG welding operation performed to lay down weld metal in the groove thus formed.

As illustrated in Figure 3, the end surfaces 21, 22 and 23, 24 are disposed angularly relative to the foundation part 16 so that they are centred on a part of the ring element 20 which will be machined away to provide the steeples shown at 13a and 13b in Figure 2.

Where turbine blades are to be secured to the discs by other than steeples of "fir tree" configuration, for example by fasteners such as rivets, the joined together end surfaces 21, 22 and 23, 24 are disposed angularly so as to permit the fasteners to be positioned at a predetermined distance from the welded together parts and in this case the welded together surfaces are fully welded together as opposed to being simply tack welded as in the first described embodiment.

After the ring element has been welded to the foundation part any necessary machining operation is performed such as the making of the steeples, and/or a final external configuration.

In the present example the discs comprise an ASTM A470 Class 8 alloy comprising

C(%) Ski(%) Mn(%) Cr(%) Ni(%) P(%) 5(%) Mo(%) V(%) Test Rotor 0,31 0,18 0,75 1,04 0,11 0,15 0,012 1,14 0,24 ASTM A470 0,25- 0,15- max. 0,90- max. max. max. 1,00- 0,20 Class 8 0,35 0,35 1,00 1,50 0,75 0,015 0,018 1,50 0,30 and the ring element is made of the same material. If desired, however, the ring element may be made of any other suitable material.

The weld material is of course compatible with the materials to be welded together.

In the above described example the extent of metal removed is that necessary to remove the damaged region and position the surface 18, as described above, at an appropriate distance from the heat affected zone and spaced from regions of relatively high stress.

If desired, however, the invention may be applied to repairing or constructing a rotor where all or substantially all or at least some of the discs are provided by a ring element.

Referring now to Figure 7, there is shown a stage in the repair of a rotor 100 having a main shaft part 101 and provided with discs 102a - d of differing diameters. In the illustrated example only two outermost discs at opposite ends of the rotor are illustrated but the rotor may be provided with any desired number of discs of suitable diameter.

At the left-hand side of Figure 7 is shown an original rotor which has had substantially the whole of its discs removed except for vestigial foundation parts 103. Ring elements 104, of the configuration shown in Figure 8, are moved radially relative to the longitudinal axis of the rotor 100 into a first position similar to that described hereinbefore and the separated end surfaces 105, 106, and 107, 108 are tack welded or otherwise joined together and then moved longitudinally of the rotor to the second position, after any desired machining operation, in a similar manner to the first described embodiment. This embodiment is similar to the previously described embodiment so further discussion is not required, the essential difference being the radial extent of the disc which is removed and replaced by the ring element.

At the right-hand side of Figure 7 is shown a version in which all of the original discs have been removed.

Although in the example illustrated in Figure 8 the separated end faces 105, 106, and 107, 108 lie on opposite sides of a diametral line, if desired the end faces may be otherwise configured, for example they may be disposed on parallel tangents to the central opening 109, as illustrated at 110 and 111 in Figure 9.

The end faces of the ring element of the first embodiment may be similarly configured and the ring elements of both embodiments may be of other configuration.

Although in both the above described embodiments the ring elements have been described as having two discontinuities to provide two pairs of separated faces, if desired, the ring element may be provided with more than two discontinuities to provide a corresponding number of pairs of separated faces or may be provided only with a single discontinuity to provide a single pair of separated faces.

Such a ring element is shown at Figure 10 where it will be seen that the ring element 20 is relatively thin. In this case the ring element is passed over larger diameter discs or foundation parts (for example, disc 12k) than that with which it is to be secured (for example, disc 12# by springing the end faces of the single discontinuity apart taking advantage of inherent elasticity of the material until the surfaces have been separated sufficiently far to enable the internal surface of the ring element to pass over the appropriate larger diameter discs or foundation parts (for example, disc 12k). This is a technique which is more suitable for ring elements of relatively small radial extent. In this case the ring elements are moved into the first position by moving the ring element axially of the rotor.

Figure 11 shows an alternative configuration of ring element provided with two discontinuities but disposed not at opposite sides of a diameter, but as shown so that a segment of the ring element shown at S is separable from the remainder of the ring element to provide a gap G of a width slightly greater than the diameter of the shaft part 11 to enable the remainder R of the ring element 20 to be introduced into the above mentioned first position by movement transverse to the longitudinal axis L-L of the rotor and then being moved axially to the second position whereat, or prior to being moved to the second position, the sector part S is mechanically inter-engaged with the remainder R by virtue of the stepped mechanical inter-engagement shown at M. If desired, the mechanical inter-engagement may be made permanent by a subsequent welding operation.

The ring elements of relatively large radial extent, such as those illustrated in Figures 8 and 9, may be utilised to make a rotor initially, that it to say by taking a rotor shaft unprovided with any discs or with only vestigial disc and attaching discs thereto or completing discs using ring elements as described hereinbefore. Alternatively, the technique described hereinbefore may be utilised to apply a further disc to an existing rotor, should the need arise.

Figure 7 may be regarded as illustrating such an originally manufactured roller where the shaft part 101 is made to the configuration shown on the right-hand side of Figure 7 or provided with vestigial discs 103 as shown on the left-hand side of Figure 7 during original manufacture.

Referring now to Figure 12, the present invention may be used to modify an existing rotor where it is desired, for example, to modify the configuration of the outer part of the rotor. For example, to modify the shape from the shape shown in dotted line in Figure 12 to that shown in full line in Figure 12. In this case a part 220 of the original rotor disc is mechanically removed and a ring element 220' of the desired modified configuration is welded to the foundation part 216 of the rotor. The foundation part itself is built up by a suitable welding operation such as a multi-wired TIG welding operation to a desired modified configuration, for example, to the configuration shown in chain dotted line at C in Figure 12. This building up operation of the foundation part 216 may be performed before or after the ring element 220' is welded to the foundation part 216.The foundation part 216 and/or the ring element 220 may be provided with a suitable weld preparation such as any of those described hereinbefore. The ring element 220' may be of any of the configurations described hereinbefore. Preferably, the ring element 220' is provided to the final or substantially the final modified configuration so as to minimise machining in situ. If desired the built up foundation part 216 may be modified to a desired shape.

Although in the example described hereinbefore the welding of the ring element to the foundation part has been described as being by a TIG welding operation, if desired it may be performed by other suitable welding operations such as a submerged arc welding operation. In this case the weld preparation may be of the configuration shown in Figure 13 and the welding operation being performed with the longitudinal axis L-L of the rotor vertical. If desired the butted together parts of the weld preparation, shown at 1 in Figure 13, may be welded together by a TIG weld prior to completing the welding operation with a submerged arc operation in the region shown at 2 in Figure 13.

The usual non-destructive examination techniques are, of course, performed on the welds made utilising the method described hereinbefore, for example by carrying out X-ray analysis of the welds, particularly those at the joined together end faces of the ring parts or ultrasonic analysis.

In the examples illustrated and described hereinbefore, the machining and welding operations are performed on site utilising a lathe which is transported to site and in which the rotor is rotated about a horizontal axis.

If desired, however, the welding operation may be performed by rotating the rotor about a vertical axis and may, if desired, be done on site or off site.

Although a rotor with individual discs for each set of turbine blades has been illustrated and described the invention is applicable to a rotor having a "disc" of relatively greater axial extent and supporting a plurality of sets of blades.

In this case an appropriate part or parts of this "disc" surface can be removed etc.

and a desired removable ring element, welded on in a similar manner to that described hereinbefore. Such a rotor is shown in Figure 14.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in the terms or means for performing the desired function, or a method or process for attaining the disclosed result, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS:

1. A method of repairing, modifying or making a turbine rotor having a plurality of blade supporting discs disposed at spaced positions along a longitudinal axis of the rotor comprising, providing a ring element having at least one discontinuity therein to provide at least one pair of separated end faces, moving said ring element into a position adjacent a foundation part of the rotor and welding the ring element to the foundation part.

2. A method according to Claim 1 wherein the separated end faces of said at least one pair of separated end faces are subsequently joined together.

3. A method according to Claim 2 wherein said separated end faces are joined together by welding.

4. A method according to Claim 2 wherein said separated end faces are joined together by mechanical inter-engagement.

5. A method according to Claim 2 wherein said separated end faces are joined together by a fastening means.

6. A method according to any one of the preceding claims wherein the ring element is initially moved to a first position adjacent the foundation part and axially displaced therefrom, said ring element being subsequently moved to a second position adjacent the foundation part and axially aligned therewith, said ring element being welded to the foundation part at said second position.

7. A method according to Claim 6 wherein said separated end faces are joined together at said first position.

8. A method according to Claim 6 wherein the separated end faces are joined together at said second position.

9. A method according to any one of the preceding claims wherein said ring element has two discontinuities so as to provide two pairs of separated end faces.

10. A method according to Claim 9 wherein said ring element is moved into a first position by movement of the ring element relative to the rotor in a direction transverse to the longitudinal axis of the rotor and moved to a second position in which the element is welded to the foundation part along the longitudinal axis of the rotor.

11. A method according to any one of the preceding claims wherein the ring element is disposed in said position adjacent to and axially aligned with the foundation part but separated therefrom by spacer members disposed between the ring element and the foundation part.

12. A method according to Claim 11 wherein said spacer members comprise roller elements.

13. A method according to any one of Claims 1 to 10 wherein the ring element is disposed in said position adjacent to and axially aligned with the foundation part and located relative thereto by virtue of engagement of weld preparation surfaces on the ring element and/or on the foundation part.

14. A method according to any one of the preceding claims wherein the ring element is made by a forging technique.

15. A method according to any one of Claims 1 to 13 wherein the ring element is made by a rolling technique.

16. A method according to any one of the preceding claims wherein the ring element is machined to a final configuration after welding to the foundation part.

17. A method according to any one of the preceding claims wherein the end surfaces of the or each pair of separated end surfaces are machined to provide a blade retaining formation.

18. A method according to any one of Claims 1 to 16 wherein turbine blades are to be retained relative to said ring by means of fasteners and wherein said separated end surfaces are disposed at a predetermined distance from the position at which said fasteners are to be provided.

19. A method according to any one of the preceding claims wherein said end surfaces are joined together by a tungsten inert gas welding process.

20. A method according to any one of the preceding claims wherein the welding of the ring element foundation part is performed by a tungsten inert gas welding operation and/or a submerged arc welding operation performed from one side of the ring element or from both sides with appropriately shaped weld preparation surfaces provided on the foundation part and/or the ring element.

21. A method according to any one of the preceding claims wherein the position of the join between the ring element and the foundation part is disposed in a region of relatively low stress.

22. A turbine rotor having a plurality of blade supporting discs disposed at spaced positions along the longitudinal axis of the rotor and wherein at least one disc comprises a ring element having at least one discontinuity therein to provide at least one pair of end faces welded to the foundation part of said rotor.

23. A turbine rotor, repaired, modified or manufactured according to the method as claimed in any one of Claims 1 toy6. Z I

24. A method of repairing, modifying or manufacturing a turbine rotor substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

25. A turbine rotor, repaired, modified or manufactured substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

26. A method of repairing, modifying or manufacturing a turbine rotor including any novel step or novel combination of steps disclosed herein and/or illustrated in the accompanying drawings.

27. A turbine rotor, repaired, modified or manufactured including any novel feature or novel combination of features disclosed herein and/or illustrated in the accompanying drawings.